can't installa Octave 4 on Debian jessie

[Francesco Potortì]

This is a strange thing.  I am used to Debian and usually I know how to
deal with small problems, but this one baffles me.

I installed a fresh stable Debian (jessie) inside a Virtualbox.  I added
the jessie-backports repository.  I updated the list of packages, but
4.0.0-5~bpo8+1 does not show up among the available packages
(octave-3.8.2 is there).

Yet, I can see it in the Packages.gz of that repository.  However, when
doing an update on the command line, I see that from jessie-backports
only "Packages/DiffIndex" files are downloaded, but no "Packages".

Could that be the reason?  What is the source of the problem?

I am going to download the .deb files and install from those, but I
would like a definitive solution.  Anyone can help?

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